Under ice plankton and lipid dynamics in a subarctic lake

Abstract

Climate warming causes shorter winters and changes in ice and snow cover in subarctic lakes, highlighting the need to better understand under-ice ecosystem functioning. The plankton community in a subarctic, oligotrophic lake was studied throughout the ice-covered season, focusing on lipid dynamics and life history traits in two actively overwintering copepods, Cyclops scutifer and Eudiaptomus graciloides. Whereas C. scutifer was overwintering in C-IV to C-V stage, E. graciloides reproduced under ice cover. Both species had accumulated lipids prior to ice-on and showed a substantial decrease in total lipid content throughout the ice-covered period: E. graciloides (60%-38% dw) and C. scutifer (73%-33% dw). Polyunsaturated fatty acids of algal origin were highest in E. graciloides and declined strongly in both species. Stearidonic acid (18:4n-3) content in E. graciloides was particularly high and decreased rapidly during the study period by 50%, probably due to reproduction. The copepods differed in feeding behavior, with the omnivore C. scutifer continuing to accumulate lipids until January, whereas the herbivorous E. graciloides accumulated lipids from under-ice primary production during the last months of ice-cover. Our findings emphasize the importance of lipid accumulation and utilization for actively overwintering copepods irrespective of the timing of their reproduction.

Keywords: Arctic; copepods; fatty acids; lake ice; phytoplankton; winter ecology; zooplankton.

Fig. 1

Data from CTD profiles (A) Temperature, (B) Chlorophyll fluorescence and (C) Oxygen saturation and (D) the moored light sensor measuring PAR. CTD data have been interpolated over the study period (dotted lines indicate actual observations from sampling dates), while for light measurements, the line shows continuous light measurements directly below ice while dots show light measurements directly above ice.

Fig. 2

(A) Chlorophyll-a concentrations in integrated 0–10 m samples and directly below the ice (0 m)., In October and November, no surface samples were taken since there was no ice cover and the lake surface waters were well mixed. (B) Rotifer species composition (full species names; Keratella cochlearis, Kellicotta longspina, Keratella hiemalis, Asplancha priodonta, Lepidurus arcticus, Filinia lonsiseta, Conochilus unicornis). (C) Zooplankton biomass (including rotifers) for samples collected from 0–20 to 0–60 meters, biomass is reported as the dry weight per vertical meter hauled. (D) Phytoplankton (n cells per liter) for samples collected from 10–0 m. Gray shaded areas in all plots indicate the ice-covered period.

Fig. 3

The abundance of Eudiaptomus graciloides (top), Cyclops scutifer (middle) and nauplii (bottom) from 0–20 to 0–60 m. Abundances are reported as number of individuals per vertical meter hauled, to facilitate comparison between 0–20 and 0–60 m hauls. For copepodite and adult stages (top and middle plots), abundances are reported by development stage. For E. graciloides it was possible to further distinguish between male adults, females carrying and egg sack (“Female + eggs”) and females without eggs (“Female”). Gray shaded areas indicate the ice-covered period.

Fig. 4

Fatty acid content for biomarker groups for zooplankton and POM. Y-axis shows the fatty acids in μg mg⁻¹ dry weight for zooplankton and μg L⁻¹ for POM (seston). BactFA, Bacterial fatty acid biomarkers; MUFA, monounsaturated fatty acids; PUFA, polyunsaturated fatty acids; SAFA, saturated fatty acids; TerFA, terrestrial fatty acid biomarkers. The line is a smoother using the loess method, the confidence interval (95%) is visualized by the light-colored area around the lines.

Fig. 5

Contribution biplot of CA of the fatty acid profiles of Cyclops scutifer (% contribution to total FA content). Individual zooplankton samples are shown as points, with color representing sampling date. FA labels (black) are included when they contribute more to the inertia than expected if all FA were equal, with the exception of C20:5n-3 that was added manually to allow for evaluation of the contribution of this important PUFA.

Fig. 6

Contribution biplot of CA of the fatty acid profiles of Euaugaptilus graciloides (% contribution to total FA content). Formatting of points, colors and labels is the same as for Fig. 5.